Update CI config to use Python 3.7

Ref: speced/bikeshed@c6b80b5

Author: xfq

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: python
 python:
-- '2.7'
+- '3.7'
 install:
 - pip install pygments lxml setuptools --upgrade
 - git clone https://github.com/tabatkins/bikeshed.git
@@ -12,4 +12,4 @@ script:
 env:
   global:
   - ENCRYPTION_LABEL: "824bd24314c2"
-  - COMMIT_AUTHOR_EMAIL: "[email protected]"
+  - COMMIT_AUTHOR_EMAIL: "[email protected]"